Vehicles are used for transporting people and cargo over land, water and through the air. Vehicles designed for land travel employ various mechanisms to apply steering and driving forces to the ground. Land vehicles may broadly be classified as wheeled, tracked, railed or skied, depending on the mechanism used to apply the steering and driving forces. A wheeled vehicle, for example, may employ wheel assemblies rotatably mounted on an axle attached to the vehicle. The wheel assembly enables the vehicle to move through rotation of the wheel assembly about the axle. A driving force may be applied to one or more of the wheel assemblies to propel the vehicle. The driving force may be generated by a power source, such as an internal combustion engine or electric motor.
Wheel assemblies for use on land vehicles are designed to handle the steering and driving forces normally encountered when operating the vehicle, and to absorb impact forces that may occur as the wheel assembly travels over imperfections in the road surface. The wheel assemblies may include a pneumatic tire mounted on a rim of a wheel. The wheel may be attached to a hub on a vehicle axle.
The tire may be pressurized with air or another gas, such as nitrogen, to provide buoyancy for supporting the vehicle load and cushioning for absorbing shocks caused by imperfections in the road. Air for inflating the tire may be introduced through a valve stem attached to an opening in the wheel adjacent the rim. The valve stem may include a valve that may be opened to admit pressurized gas (i.e., air) to an interior region of the tire, and which automatically closes and is kept sealed by the interior tire pressure, a spring, or both, to prevent the gas from escaping. The valve stem may include a flexible grommet that seals the valve stem against the wheel to prevent pressurized gas from escaping the tire. The valve stem may be assembled to the wheel by inserting the valve stem through the valve stem opening in the wheel from inside the wheel.
The tire may be inflated to a recommended pressure selected to provide a desired balance between vehicle performance and handling, ride quality, and tire life. For example, an underinflated tire may provide a vehicle operator with a softer ride by decreasing tire stiffness, but may also negatively impact vehicle handling and fuel mileage. Under-inflation may also result in premature tire wear and diminished tire performance in certain operating conditions. Overinflating a tire, on the other hand, may increases tire stiffness and result in a firmer ride. Over-inflation may also have a detrimental effect on tire life and result in decreased tire performance under certain operating conditions.
To monitor and help maintain a tire at a recommended inflation pressure, and to alert a vehicle operator of an extreme under-inflation condition, a tire pressure monitoring system (TPMS) may be employed to automatically monitor the inflation pressure. A TPMS electronically monitors a tire inflation pressure. A TPMS may also be referred to as a tire-pressure indication system (TPIS). A TPMS reports real-time tire pressure information to a vehicle operator, typically via a gauge, a pictogram display, or a simple low-pressure warning light. A TPMS may employ a pressure sensor mounted internally within the tire for measuring the tire inflation pressure. The collected pressure data may be transmitted to the vehicle's instrument cluster or a corresponding monitor for viewing by the vehicle operator. Data may be transmitted from the pressure sensor to the monitor or display via a battery powered radio-frequency (RF) communication device. The pressure sensor is generally fitted to the wheel or to the in-tire portion of the valve stem when the tire is mounted to the rim of the wheel.